1. Field of the Invention
This invention concerns optical fibers having one or more coated layers thereon.
2. Description of the Prior Art
Optical fibers require protective coatings in order to preserve fiber strength and to protect the fiber from microbending induced optical loss. The coating is generally applied in-line with fiber drawing by passing the fiber through a reservoir containing the coating material and having an exit orifice which has been sized to apply some desired thickness of the material. This technique usually requires the coating material to have a viscosity below approximately 10,000 centipoises at the application temperature if high draw rates, 1 meter/second or higher, are desired. Classes of materials which have been applied to optical fibers by this technique include ultraviolet (UV) curables, thermal curables, solvent-based materials, and hot melts. For an example of the latter, see "Silicone-and-Ethylene-Vinyl-Acetate-Coated Laser-Drawn Silica Fibres with Tensile Strengths &gt;3.5 GN/m.sup.2 (500 kp.s.i.) in &gt;3 km Lengths," T. J. Miller et al, Electronics Letters, Vol. 14, No. 18, pages 603-605 (1978). The "curable" materials are applied as liquids, and solidify by polymerization, which is typically accelerated by ultraviolet radiation or by heating. The solvent-based materials solidify by the removal of the solvent, which again can be accelerated by heating. In contrast, "hot melt" materials have low viscosity at high temperature and solidify upon cooling.
Properties of the coating material which influence the ability to preserve fiber strength are its toughness, abrasion resistance, adhesion to the fiber, and thickness. In general, an increase in any of these properties reduces the susceptibility of the fiber to mechanical damage. In addition, low water absorption by the coating is desirable to preserve fiber strength.
The property of the coating material which correlates most closely to microbending sensitivity is modulus. In general, a reduction in the coating modulus decreases microbending sensitivity. The temperature range over which a low modulus is required depends upon the environment the fiber is expected to experience. For the temperature range -40 to 90 degrees Celsius, the 30 minute tensile relaxation modulus should typically be less than 10.sup.8 dynes/cm.sup.2. This wide operating temperature range for the coated fiber represents expected extremes of service conditions. Many prior art optical fiber coating materials have not obtained good microbending performance over the above wide temperature range.
Thus, in the past, thermally cured silicones with a modulus below 10.sup.7 dynes/cm.sup.2 over the expected operating temperature have been used as coating where low sensitivity to microbending is required. Unfortunately, a low modulus also often means insufficient toughness and abrasion resistance for adequate strength protection of the fiber. Such is frequently the case with the silicones. For this reason, a secondary coating is often placed over the primary silicone for strength protection.
At present, typical silicones also have process limitations in terms of preapplication stability and capability for application at high fiber draw rates, i.e., &gt;1 meter/second, due to their curing behavior. For example, a silicone formulation typically comprises two components that begin to react upon mixing. They further react more rapidly as the hot fiber, drawn at high temperature from a preform, transfers heat to the bulk coating material in the reservoir. As the reaction continues, the viscosity of the silicone increases, ultimately reaching an unusable value if not applied soon enough to the fiber. Thus, the useful lifetime in the applicator is limited, typically being about 2 hours. This typically limits the amount of material that can be maintained in the mixed state, requiring frequent replenishment of the applicator, and sometimes results in an interruption of the coating process.
Another problem concerns the ability to transfer heat to the silicones rapidly enough to cure and harden before the fiber contacts the guide wheel and pulling apparatus. This hardening is necessary to preserve the strength of the fiber and maintain coating quality. Typically with a silicone coating, a furnace is placed below the coating applicator to accelerate the curing process. As draw rates increase, longer furnaces are required, which in turn, require higher draw towers, an expensive solution. The problem is compounded with dual-layer coatings, wherein both coatings must harden in the time and space available, and with the necessity that the first coating harden sufficiently prior to applying the second coating. While ultraviolet-cured silicone materials are currently under investigation by workers in the art, it presently appears that they are not yet ready for commercial application, especially when high cure rates are desired for high-speed fiber production. It is thus desirable to obtain other optical fiber coating materials having good microbending performance over a wide temperature range, and having processing characteristics suitable for high-speed fiber manufacture.